1. Field of the Invention
The invention relates to means which set an initial value for an electronic device comprising a RAM which stores information such as time data or others, a ROM which stores the program of an operation process for various functions, and LSI composed of a CPU for logical operation.
2. Description of the Prior Art
Various countermeasures to prevent malfunctioning of an electronic device by external disturbance noises, such as power source fluctuation, have been adapted to the conventional electronic apparatus employing C-MOS LSI: that is, in order to absorb surge-noise generated in a power source, a large capacitance is connected in a row with power source batteries; and in order to absorb disturbance noise, the relation of positions between respective input/output terminals and power source on the above LSI are regulated, and capacitance elements are added between said input/output terminals and power source.
When power is turned on, it is necessary to set an initial value in order to operate electronic equipment employing LSIs which are constructed with ROM, RAM and CPU. In the operation for setting an initial value of electronic timepieces, for example, by storing data of initial values such as counting and storing functions for the time or others as the procedure for setting an initial value in a ROM which stores the program for operation procedure of various functions, and by executing the operation of an initial value setting means, the data of various functions stored in a RAM are initialized. Therefore, the conventional electric device had a defect of causing an erroneous operation of all-clear during normal operation when the above initial value setting means is operated by disturbance noise. In addition, since it is only to decrease power source noise generated by disturbance noises in the case of the abovementioned countermeasures employed in the prior art, such countermeasures could not work effectively.
As one of the examples with respect to said disturbance noises, a well-known operation of a power-on clear circuit during power source change will now be described. FIG. 2 shows the most typical example of a power-on clear circuit which is one element of the system clear means 9 shown in FIG. 1. The power-on clear circuit of FIG. 2 comprises a resistor R composed of PchTr 10 which joins a source to Vdd and a gate to Vss respectively. A condenser 11 is connected in series with the resistor R, and an inverter (INV) 12 is connected to the middle point "a" between said C and R. FIG. 3 is a time chart which shows that a pulse is generated by the Power-on Clear Circuit when Vdd is changed by a disturbance noise. Waveforms at points "a" and "b" exist in response to changes in the power source (indicated by dashed line) on the power-on clear circuit shown in FIG. 2. A dash and dotted line in FIG. 3 indicates changes of the threshold voltage VTH (VTH=VDD/2) of the inverter INV 12 when changes occur in the power source. When the power is inputted or changed, the waveform at the middle point "a" between the condenser C 11 and the resistance R 10 connected in series becomes a differential waveform as shown in a solid line in FIG. 3 due to the charging characteristics of the condenser. Since the V.sub.TH of INV 12 changes according to changes in the power source, the INV 12 outputs V.sub.DD a signal to the point "b" in case the output at point "a" is below the V.sub.TH. This signal at the point "b" results in all-clear.